Exercise spring



FIG. 1 is a perspective view of an exercise spring showing my new design;

FIG. 2 is a front view of the spring of FIG. 1 with the rear view being a mirror image thereof;

FIG. 3 is a right end view of the spring of FIG. 1 with the left end view being a mirror image thereof;

FIG. 4 is a top view of the spring of FIG. 1; and,

FIG. 5 is a bottom view of the spring of FIG. 1. 

The ornamental design for an exercise spring, as shown and described. 